Friday, August 20, 2010

If packet N+1 of a TCP flow arrives before packet N, the receiving application does not see any data until packet N gets there. That's what we mean when we say TCP guarantees in-order delivery. That is also true if N+1 through N+100 get there before N - nobody gets through until they can all be delivered in-order.

At least using the BSD socket API.

I got to thinking about the impact of this when discussing a multiplexing implementation of various logical streams on top of TCP. For instance, SPDY and BEEP do things along those lines in order to create efficiencies in terms of more accurate congestion control data. But as someone objected, that creates a certain amount of fate sharing between the different streams that wouldn't exist if they were on separate TCP channels. A packet loss in one of them creates a delay for them both even though throughput might very well be maintained using some variation of fast-retransmit and large windows.

So the question: how often are packets received but the data in them is delayed by he kernel because the stream isn't yet in order? And how long are those delays?

I don't know yet, but I wrote some crude linux kernel patches to find out. When a skb is moved out of the out of order queue a structure with 2 timestamps (in queue, out of queue) is passed to userspace through the netlink connector mechanism. It also reports the total number of received data packets on each TCP stream. That way we can find out, how often and how long.